An important aspect of an electrodeposition coating system is its throw power. The term throw power refers to the ability to electrodeposit coatings in recessed areas of a work piece. A system which has the ability to coat highly recessed areas is said to have high throw power. High throw power systems are desirable because a work piece can be more completely coated. For example, in automotive applications, coating of interior surfaces of double walled work pieces is desirable for increased corrosion resistance. Similarly, in the electrodeposition of other industrial articles, such as heaters or radiators having multiple walls or panels, high throw power electrodeposition systems are necessary to provide more corrosion resistance.
It is known, for a given system, that throw power can be increased by the application of higher voltage. However, excessively high voltage will cause film ruptures. Thus, coatings which have a high rupture voltage are useful because higher throw power can be achieved while maintaining a smooth uniform film without ruptures. Throw power can also be affected by a higher conductivity of the electrodeposition bath. It is also generally recognized that higher molecular weight compositions tend to have higher throw power.
The present invention relates to the surprising finding that the use of gamma hydroxy urethane curing agents provide high throw power. A preferred embodiment of the gamma hydroxy urethane curing agent of the present invention is a reaction product of a polyisocyanate and a 1,3-polyol, wherein the ratio of isocyanate groups to hydroxyl groups from the 1,3-polyol is less than 1. The prior art discloses curing agents which are distinguishable from the curing agent of the present invention, but which are also formed from 1,3-polyols.
U.S. Pat. No. 4,225,478 (Hicks 1980) discusses the use of a blocked isocyanate made from polyphenyl isocyanate having an average functionality of 2.4 blocked with 0.8 equivalents of caprolactam and 0.3 equivalents of 2,2,4-trimethyl-1,3-pentanediol per isocyanate equivalent. Because the isocyanate groups are in excess, both hydroxyl groups on the 2,2,4-trimethyl 1,3-pentane diol will be reacted in such a formulation. Examples 9 and 10 in U.S. Pat. No. 4,134,864 (Belanger 1979) and examples 8 and 9 of U.S. Pat. No. 4,139,510 (Anderson 1979) disclose similar uses of 2,2,4-trimethyl-1,3-pentanediol.
U.S. Pat. No. 4,748,200 (Nasu 1988) discloses the use of a thermosetting resin which contains .alpha.,.alpha.,.alpha.',.alpha.'-tetramethylxylene diisocyanate (TMXDI) and a prepolymer of TMXDI in which the isocyanate groups are reacted with an active hydrogen-containing compound. 2,2,4-trimethyl-1,3-pentanediol is disclosed as one optional active hydrogen-containing compound. When reacted together as taught in Nasu, however, the isocyanate groups are in excess over the active hydrogens of the active hydrogen-containing compounds. Therefore, both hydroxyl groups of the diol will be reacted with isocyante groups.
Compounds which are structurally similar to the gamma hydroxy urethane of the present invention are also known. For example, U.S. Pat. No. 4,435,559 (Valko 1984) discloses a beta hydroxy urethane curing agent which is effective at low temperatures. Such compositions, however, do not provide the surprising effect of increased throw power as compounds of the present invention do.
It has been observed that some blocked isocyanate containing electrodeposition baths have high throw power at very high resin particle sizes. However, this approach is not commercially useful because at higher particle size, resins tend to be unstable either upon standing or under shearing conditions. Another disadvantage with some of such known blocking agents, such as higher aliphatic blocking agents, is an unacceptably high cure temperature of up to between 360.degree. F. and 400.degree. F.